A Review on Thermoelectric Generators: Progress and Applications

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Review A Review on Thermoelectric Generators: Progress and Applications

Mohamed Amine Zoui 1,2 , Saïd Bentouba 2 , John G. Stocholm 3 and Mahmoud Bourouis 4,* 1 Laboratory of Energy, Environment and Information Systems (LEESI), University of Adrar, Adrar 01000, Algeria; [email protected] 2 Laboratory of Sustainable Development and Computing (LDDI), University of Adrar, Adrar 01000, Algeria; [email protected] 3 Marvel Thermoelectrics, 11 rue Joachim du Bellay, 78540 Vernouillet, Île de France, France; [email protected] 4 Department of Mechanical Engineering, Universitat Rovira i Virgili, Av. Països Catalans No. 26, 43007 Tarragona, Spain * Correspondence: [email protected]

Received: 7 June 2020; Accepted: 7 July 2020; Published: 13 July 2020

Abstract: A thermoelectric effect is a physical phenomenon consisting of the direct conversion of heat into electrical energy (Seebeck effect) or inversely from electrical current into heat (Peltier effect) without moving mechanical parts. The low efficiency of thermoelectric devices has limited their applications to certain areas, such as refrigeration, heat recovery, power generation and renewable energy. However, for specific applications like space probes, laboratory equipment and medical applications, where cost and efficiency are not as important as availability, reliability and predictability, thermoelectricity offers noteworthy potential. The challenge of making thermoelectricity a future leader in waste heat recovery and renewable energy is intensified by the integration of nanotechnology. In this review, state-of-the-art thermoelectric generators, applications and recent progress are reported. Fundamental knowledge of the thermoelectric effect, basic laws, and parameters affecting the efficiency of conventional and new thermoelectric materials are discussed. The applications of thermoelectricity are grouped into three main domains. The first group deals with the use of heat emitted from a radioisotope to supply electricity to various devices. In this group, space exploration was the only application for which thermoelectricity was successful. In the second group, a natural heat source could prove useful for producing electricity, but as thermoelectricity is still at an initial phase because of low conversion efficiency, applications are still at laboratory level. The third group is progressing at a high speed, mainly because the investigations are funded by governments and/or car manufacturers, with the final aim of reducing vehicle fuel consumption and ultimately mitigating the effect of greenhouse gas emissions.

Keywords: thermoelectric generator; ﬁgure of merit; thermoelectric materials; nanostructuring

1. Introduction The supply of healthy and non-polluting energy is one of today’s major concerns. Fossil fuels currently make up the largest contribution to global energy production. These energy sources are polluting, they emit greenhouse gases and, furthermore, will run out in a few decades’ time [1]. The only current competitor is nuclear power, but the fatal risks involved in nuclear operation, as seen in the nuclear accident at the Fukushima Daiichi power plant (Japan) in March 2011, have limited any expansion or development in the nuclear sector [2]. It is essential for future generations to reduce the quantity of global energy consumed, and this can only be achieved through technological development and the use of diversiﬁed renewable energy

Energies 2020, 13, 3606; doi:10.3390/en13143606 www.mdpi.com/journal/energies Energies 2020, 13, 3606 2 of 32 sources, i.e., solar, wind and hydropower, in addition to the energy sources currently used [3]. Among these different energy sources, thermoelectricity is currently emerging as a common and promising alternative energy source for the future [4]. Thermoelectric materials have the specific capacity of converting a flow of heat into electrical energy (Seebeck effect) and vice versa (Peltier effect) [5]. Their use is becoming of more interest, as they offer the advantages of recycling waste energy. This means transforming the heat from industry or road transport into electricity, thus increasing system efficiency and decreasing operating costs and environmental pollution. To cite an example of the heat involved, the temperature of exhaust gases emitted from vehicle engines, biomass combustion systems and matrix-stabilized porous medium combustion can reach 500 ◦C, while the operating temperature of micro-turbine power cycles can rise to 600 ◦C, and even to 900 ◦C in the case of a solar energy receiver [6]. Thermoelectric devices are particularly reliable, silent, and do not generate vibrations since their operation does not require the contribution of mechanical energy [7]. For these reasons, major efforts have been carried out, using new materials, to develop the technology of thermoelectric systems. It was during the 1960s that the most important research programs, using semiconductor materials, were carried out in this field [8]. Since the discovery of thermoelectricity (TE) in 1821 by Seebeck [9], researchers have been trying to understand and control this phenomenon. Peltier did exactly this in 1834 by discovering the opposite effect [10], and Lord Calvin in 1851 formulated the laws that link these two phenomena [11]. In the following century, in 1909, Edmund Altenkirch [12] correctly calculated, for the first time, the energy efficiency of a thermoelectric generator now known as figure of merit (ZT), and two years later that of thermoelectricity in cooling mode [13]. In 1912, Altenkirch invented a thermoelectric heating and cooling apparatus [14], which was later succeeded by several other prototypes developed by various scientists and companies. Unfortunately, these attempts to produce a practical refrigerator failed due to the lack of suitable materials [15]. It is worth note that a ZT > 3 for refrigeration and ZT > 2 for power generation is required to replace a conventional energy system [16]. In 1950, Abram Ioffe discovered the thermoelectric properties of semiconductors [17], which opened up new projections for thermoelectricity with a figure of merit ZT close to 1. This value was still low, but acceptable enough for some inventors and industrialists to design new applications to be commercialized. One such application was the thermoelectric refrigerator designed by Becket et al. in 1956 [18]. In the same decade the idea of thermoelectric generators emerged, such as Ioffé’s thermoelectric lamp in 1957, which fed a radio by recovering the heat released by the lamp. In 1993, Hicks and Mildred Dresselhaus [19] showed that quantum-well superlattice structures (small dimensions of matter) could affect thermoelectricity by reducing phonon thermal conductivity, and therefore improving the ZT by a factor of 13. As a result, a new era in thermoelectricity was launched and gave rise to an exponential increase in the number of research projects being carried out into thermoelectricity (Figure1)[20]. EnergiesEnergies2020 2020, ,13 13,, 3606 x FOR PEER REVIEW 33 of of 32 34

Figure 1. Open literature publications on the Web of Science database with the keyword “thermoelectric” asFigure a percentage 1. Open of allliterature publications publications on the database on the for Web each of year Science from 1955 database to 2003 with [20]. the keyword “thermoelectric” as a percentage of all publications on the database for each year from 1955 to 2003 2. Thermoelectric[20]. Modules To extend the use of thermoelectricity, it was essential to manufacture standard thermoelectric modules2. Thermoelectric of different Modules sizes, accessible to all. In 1959 the General Electric company commercialized [21] thermoelectricTo extend modules the use composedof thermoelectricity, of 36 couples it ofwas bismuth essential telluride to manufacture in flat bulk architecture.standard thermoelectric Nowadays, theremodules are dozensof different of companies sizes, accessible all over to the all. world In 1959 that the manufacture General Electric TE modules. company Some commercialized of these are listed[21] thermoelectric in [22]. modules composed of 36 couples of bismuth telluride in flat bulk architecture. Nowadays,A typical there thermoelectric are dozens generatorof companies (TEG) all moduleover the consistsworld that of between manufacture 10 and TE 100 modules. thermoelectric Some of elementsthese are of listed type in n and[22]. type p, electrically connected in series and thermally in parallel, and interposed betweenA typical two ceramic thermoelectric layers, generator as shown (TEG) in Figure module2. The consists p–n pairs of between are joined 10 and by 100 conductive thermoelectric tabs connectedelements toof thetype elements n and viatype a low p, meltingelectrically point connected solder (PbSn in series or BiSn). and When thermally a temperature in parallel, gradient and occursinterposed between between its two two junctions, ceramic thelayers, TEG as converts shown in thermal Figure energy2. The p–n into pairs electrical are joined energy by according conductive to thetabs principle connected of theto the Seebeck elements effect. via This a low flat melting bulk architecture point solder is the(PbSn most or widelyBiSn). When used anda temperature marketed. However,gradient occurs in some between applications its two a flat junctions, shape is notthe practical.TEG converts This isthermal because energy of the diintofficulty electrical in adapting energy theaccording heat source to the to theprinciple thermoelectric of the Seebeck device, whicheffect. makesThis flat it more bulk costly,architecture heavier, is and the moremost cumbersome. widely used Asand a result,marketed. other However, designs are in being some investigated applications to a overcome flat shape these is not drawbacks, practical. althoughThis is because most of them,of the likedifficulty the cylindrical in adapting shape the [23 heat–25], source are still to at the laboratorythermoelectric stage. device, This is which the subject makes of limitedit more studies, costly, unlikeheavier, thick and and more thin cumbersome. films and flexible As a result, TE devices, other designs which are are being being developed investigated more to overcome effectively these [26]. Moredrawbacks, details although on the usefulness most of them, of these like designs the cylindrical are presented shape in[23–25], the following are still sections.at the laboratory stage. ThisThe is the two subject ceramic of plateslimited serve studies, as a unlike support thic fork and the modulethin films and and as electricalflexible TE insulation, devices, which but their are thermalbeing developed resistance more degrades effectively the module’s [26]. More efficiency. details From on the this, usefulness some investigations of these designs have are proposed presented the conceptin the following of a direct sections. contact thermoelectric generator (DCTEG), which is characterized by one of the surfacesThe of two the ceramic module beingplates directlyserve as exposed a support to thefor heatthe module source and and the as otherelectrical surface insulation, in direct but contact their withthermal the coolantresistance flow degrades [27,28]. Severalthe module’s manufacturing efficiency technologies. From this, some for TE investigations modules are reportedhave proposed in the literature.the concept Some of a examples direct contact include thermoelectric foil lithography gene [29rator], the (DCTEG), lift-off process which [ 30is ],characterized flash evaporation by one [31 of], the surfaces of the module being directly exposed to the heat source and the other surface in direct contact with the coolant flow [27,28]. Several manufacturing technologies for TE modules are

Energies 2020, 13, 3606 4 of 32 Energies 2020, 13, x FOR PEER REVIEW 4 of 34 evaporationreported in the thin literature. ﬁlm [32], Some photolithography examples include and etchingfoil lithography [33], screen-printing [29], the lift-off [34 process], sputtering [30], flash [35], dispenserevaporation printing [31], evaporation [36], the spark thin film plasma [32], sinteringphotolithography technique and [37 etching], direct [33], current screen-printing (DC) magnetic [34], sputteringsputtering [[35],38] and dispenser the printing printing process [36], [39 the]. spark plasma sintering technique [37], direct current (DC) magnetic sputtering [38] and the printing process [39].

FigureFigure 2.2. Diagram ofof aa typicaltypical thermoelectricthermoelectric devicedevice [[40].40].

The critical challenge in the development of TEGs is the degradation of original properties The critical challenge in the development of TEGs is the degradation of original properties brought on by thermal fatigue, which is in turn caused by thermal expansion and thermal shock [41]. brought on by thermal fatigue, which is in turn caused by thermal expansion and thermal shock [41]. This degradation can be brutal or progressive, and result in a decrease in service life and efficiency. This degradation can be brutal or progressive, and result in a decrease in service life and efficiency. In fact, during the normal operation of TE devices, the shunts are periodically heated and cooled and In fact, during the normal operation of TE devices, the shunts are periodically heated and cooled and undergo thermal expansion. The TE materials connected to these shunts can experience different undergo thermal expansion. The TE materials connected to these shunts can experience different effects of expansion from temperature sources, which cause increased stress at the interface between effects of expansion from temperature sources, which cause increased stress at the interface between them. These stresses are generally the main cause of mechanism failure, and consequently the principal them. These stresses are generally the main cause of mechanism failure, and consequently the reason why TE materials are not sintered and integrated into shunts [42]. principal reason why TE materials are not sintered and integrated into shunts [42]. 3. Figure of Merit and Other Performance Parameters 3. Figure of Merit and Other Performance Parameters The thermoelectric efficiency of a TE material is expressed by the dimensionless thermoelectric The thermoelectric efficiency of a TE material is expressed by the dimensionless thermoelectric figure of merit (ZT), which is dependent on the transport properties of the material as shown in figure of merit (ZT), which is dependent on the transport properties of the material as shown in Equation (1). Equation (1). S2σT ZT = (1) KσT = (1) where S is the Seebeck coefficient (µV/K), σ the electricalK conductivity (1/Ωm) and K is the thermal conductivitywhere S is the (W Seebeck/mK). This coefficient equation (µV/K), shows σthat, the electrical to maximize conductivity the ZT of (1/ a material,Ωm) and itK mustis the meet thermal the followingconductivity criteria: (W/mK). (i) lowThis thermal equation conductivity shows that, toto maintainmaximize a the considerable ZT of a material, temperature it must di meetfference the betweenfollowing the criteria: two ends (i) oflow the thermal material; conductivity (ii) high electrical to maintain conductivity a considerable to reduce temperature the internal difference resistance ofbetween the material the two and consequentlyends of the thematerial; Joule eff(ii)ect; high and (iii)electrical a high Seebeckconductivity coeffi cient,to reduce required the tointernal obtain aresistance high voltage of the [43 ,material44]. Unfortunately, and consequently according the to Jo theule graph effect; of and Figure (iii)3 ,a these high parameters Seebeck coefficient, are well correlatedrequired to and obtain it is verya high di ffivoltagecult to [43,44]. optimize Unfortun them independently,ately, according especially to the graph for conventional of Figure 3, metals. these parameters are well correlated and it is very difficult to optimize them independently, especially for conventional metals.

Energies 2020, 13, 3606 5 of 32 Energies 2020, 13, x FOR PEER REVIEW 5 of 34

FigureFigure 3.3.Relationship Relationship between between ﬁgure figure of merit of ZTmerit and ZT other and parameters other parameters such as electrical such conductivityas electrical 2 σconductivity, Seebeck coe σﬃ, cientSeebeckS, power coefficient factor S,S powerσ, electronic factor thermalS2σ, electronic conductivity thermal Ke, co thermalnductivity conductivity Ke, thermal of theconductivity network Kl of andthe network total thermal Kl and conductivity total thermal K [ 16conductivity]. K [16].

InIn thethe literature,literature, ZTZT isis sometimessometimes referredreferred toto asas thermoelectricthermoelectric eefficiencyﬃciency becausebecause it isis relatedrelated toto thethe eefficiencyﬃciency ofof aa singlesingle elementelement andand thethe device,device, viavia EquationEquation (2).(2).

∆T∆ √1+ZTave 1 η = − (2) opt =T (2) h √1+ZTave+ Tc//Th wherewhere ∆ΔTTis is the the di differencefference between between the the temperature temperature on on the the warm warm side sideTh and Th and the coldthe cold side Tsidec. The Tc. firstThe termfirst ofterm this of equation this equation represents represents the Carnot the effi Carnciency.ot efficiency. To determine To thedetermine efficiency the of aefficiency thermoelectric of a generatorthermoelectric (TEG), generator it is necessary (TEG), to calculateit is necessary the ratio to between calculate the the electrical ratio between power produced the electrical and the power heat flowproduced through and the the module. heat flow These through basic the standard module. equations These basic are typically standard built equations on four are main typically hypotheses, built namelyon four (i)main the hypotheses, electrical and namely thermal (i) contactthe electrical resistances and thermal are negligible, contact resistances (ii) the Thomson are negligible, effect has (ii) a negligiblethe Thomson effect effect on e ffihasciency, a negligible (iii) the convectioneffect on ef andficiency, radiation (iii) heatthe transferconvection are and negligible, radiation and heat (iv) thetransfer dependency are negligible, on the and thermoelectric (iv) the dependency transport propertieson the thermoelectric of the TEG transport module with properties temperature, of the whichTEG module makes the with performance temperature, of TEGs which change makes at di thfferente performance temperatures of [45TEGs]. change at different temperaturesThere are [45]. several ways to calculate the performance of a thermoelectric couple in terms of energy production,There are either several averaging ways or to using calculate finite the elements performa [46,47nce]. Theof a averaging thermoelectric methods couple overestimate in terms the of eenergyfficiency production, but provide either an immediate averaging value or based using on fi thenite calculated elements properties [46,47]. The of the averaging TEG at the methods average junctionoverestimate temperature the efficiency [48]. Onbut the provide other an hand, immediat finite elementse value based require on athe lot calculated of iterations properties and therefore of the takeTEG moreat the time average to obtain junction results temperature [49]. [48]. On the other hand, finite elements require a lot of iterationsAlthough and therefore simple one-dimensional take more time analytical to obtain models results are [49] frequently. used to predict the performance of suchAlthough devices [simple50], the one-dimensional diversity and complexity analytical of thermoelectricmodels are frequently applications used generally to predict require the a completeperformance three-dimensional of such devices (3D) [50], numerical the diversity analysis and [51], complexity using simulation of thermoelectric tools such as ANSYSapplications [52], GT-SUITEgenerally require [53], FLUENT a complete [54] andthree-dimensional GTPower [55]. (3D) numerical analysis [51], using simulation tools such as ANSYS [52], GT-SUITE [53], FLUENT [54] and GTPower [55]. 4. Thermoelectric Materials 4. ThermoelectricEver since the Materials discovery of TE materials, their use has been limited to thermocouples for temperatureEver since measurements the discovery due of toTE their materials, very low their effi useciency has [ 56been]. It limited was only to inthermocouples the 1960s when for relevanttemperature investigations measurements into thermoelectric due to their semiconductorsvery low efficiency developed [56]. It more was applications only in the in 1960s the field when of refrigerationrelevant investigations and power into generation. thermoelectric semiconductors developed more applications in the field of refrigerationFrom the discovery and power of generation. thermoelectric semiconductors [17] until 1993, the figure of merit (ZT) experiencedFrom the a discovery modest improvement. of thermoelectric After semiconducto that date, theoreticalrs [17] until predictions 1993, the figure suggested of merit that (ZT) the eexperiencedfficiency of TEa modest materials improvement. could be significantly After that improved date, theoretical by using predictions nanostructure suggested engineering that [19the]. Ruraliefficiency et al. of [ 57TE] theoreticallymaterials could and be experimentally significantly exploredimproved nanostructuring by using nanostructure as effective engineering in reducing [19]. the negativeRurali et correlational. [57] theoretically of thermoelectric and experimentally transport properties. explored Fornanostructuring instance, 2D andas effective 1D nanostructuring in reducing the negative correlation of thermoelectric transport properties. For instance, 2D and 1D nanostructuring slows down the diffusion of phonons, leading to a decrease in thermal conductivity

Energies 2020, 13, 3606 6 of 32 slows down the diffusion of phonons, leading to a decrease in thermal conductivity that in turn increases the efficiency of the TEG. Hence, great interest has arisen in nanostructured thermoelectric materials. At the same time, using modern synthesis and characterization techniques, conventional bulk materials containing nanostructured components were explored and developed with the aim of achieving higher yields [58]. Thus, nowadays, the ZT factor can be increased in one of two ways: (i) bulk samples containing nanomaterial constitute, and (ii) nanomaterials themselves [59]. It should be mentioned that small (nanostructured) materials are difficult to manufacture with precision due to the high sample requirements on the nanoscale [60]. Thermoelectric materials can be classified into two categories, namely conventional and new materials.

4.1. Conventional Thermoelectric Materials Conventional thermoelectric materials, which are bulk doped semiconductor alloys or chalcogenide, can be grouped into three families according to the temperature range at which the performance is optimum: Bi2Te3-based materials for ambient temperature applications (< 150 ◦C), TAGS [(AgSbTe2)1-x(GeTe)x] and PbTe-based materials for intermediate temperature range (150–500 ◦C), and SiGe for use at temperatures over 500 ◦C[61–63]. The temperature range can be widened by using a combination of materials characterized by different temperature ranges in a segmented structure [64]. The Bi2Te3 are well known and can have a ZT close to the unit at room temperature. However, as they are easily oxidized and vaporized, these materials cannot be used for high-temperature applications in air [65]. Around 70% of the TE modules available on the market use Bismuth and Telluride as functional materials [66]. Recently, Mamur et al. [67] reviewed the latest research on the growth of the Bi2Te3 nanostructure by various methods, and its characterization by theoretical and analytical approaches. The authors have concluded that the figure of merit (ZT) rises from 0.58 to 1.16 if materials are developed in a nanostructure form. Lead telluride (PbTe) is a good thermoelectric material for applications requiring mid-temperatures up to 900 K. PbTe has a high melting point of 1190 K, good chemical stability, low vapor pressure and robust chemical strength [68]. Its high figure of merit, approaching 0.8, allowed for its successful use in several NASA space missions. Recent investigations have reported maximum ZT values of around 1.4 for single phase PbTe-based materials, and 1.8 for homogeneous PbTe-PbSe materials [69]. An extensive survey of Research and Development on PbTe and its related compounds, alloys and composites, as well as PbTe-based nanostructured composites, can be found in references [70,71]. Silicon–Germanium alloys (Si1-xGex) are among the best TE materials reported in the literature for high-temperature applications (Thot side > 500 ◦C). In addition, they are one of the cheapest and most nontoxic thermoelectric materials [61]. Delime-Codrin et al. [72] reported a significant figure of merit, ZT = 1.88 at 873 K, with nanostructured Si0.55Ge0.35(P0.10 Fe0.01).

4.2. New Thermoelectric Materials Phonon-glass electron-crystal (PGEC) materials proposed by Slack [73] have a complex intermetallic cage structure, which gives the material good electronic characteristics like crystal, and at the same time, a low thermal conductivity, like glass [74]. Two categories of relatively new materials are generally considered to be PGEC materials, namely clathrates and Skutterudites [75]. Among other TE materials, half-Heusler alloys have attracted considerable attention with their attractive electrical transport properties, relatively high Seebeck coefficients and rich element combinations [76]. In addition, they boast robust mechanical strength, good thermal stability at high temperatures and multiform physical properties [77]. In 2005, Gascoin et al. [78] proposed phase Zintl as an attractive candidate for thermoelectric materials. These are typically small-bandgap semiconductors with a complex structure. Since then, several studies have been carried out on the use of this type of material, and the best values achieved for ZT ranged from 1 to a peak value of 1.5 [79]. Over the past two decades, since the discovery of the first ceramic thermoelectric material, many efforts have been made to obtain high-performance thermoelectric materials for energy conversion Energies 2020, 13, 3606 7 of 32 systems [80]. TE oxides, such as Ca Co O (ZT 1), are good TE performers [43], and are ecological 3 4 9 ∼ and essentially stable at high temperatures [81]. Other oxides that can be used at high temperatures without oxidizing have also attracted much attention in recent years [82]. The thermoelectric metal chalcogenide has high electrical properties and low thermal conductivity, so when advanced nanostructuring and band engineering are employed, the result is an improved figure of merit (ZT). Furthermore, chalcogenides are easy to process into different types of structures, thus offering huge potential for improvement in thermoelectric performance. The highest values of ZT recorded with lead selenide (PbSe) ranged from 1.4 to 1.7 at 800–900 K [83]. In the case of Tin chalcogenides Sn (Se,Te), numerical values of ZT above 2.3 were obtained at 723–973 K for single crystal SnSe [84], and about 1.6 at 923 K for SnTe-based materials [85]. A further advantage is that these materials are low-cost and operate at high and even medium temperatures. However, their low mechanical properties and low thermal stability, and in some cases the presence of toxic elements (e.g., Pb), limit their use in real applications. Since the discovery of conductive polymers, great interest has been shown in organic TE materials [86]. These are lightweight, flexible and suitable for applications at room temperature, generally with relatively simple manufacturing processes compared to other semiconductor-based materials. Polymers are intrinsically poor thermal conductors, which makes them suitable for use in thermoelectricity, but their low electrical conductivity, Seebeck coefficient and stability have limited their use in thermoelectric applications [87]. However, compared to inorganic TE materials, organic or polymeric TE materials boast several advantages, such as a potentially low cost due to the abundance of carbon resources, and a quite simple synthesizing process. In addition, the physical and chemical properties of some polymers can be subjected to a fairly wide range of modifications in their molecular structures [88]. The properties of polymers and polymer-based TE composites have been significantly improved with ZT values up to 0.42 [89]. It is worthy of note that the hybridization method, which consists of mixing all the thermoelectric materials already mentioned, has also achieved results [90]. Another material, Graphene (carbon atoms forming a crystalline two-dimensional material), has attracted considerable interest since its discovery in 2004 because it has many unusual thermoelectric and thermal transport properties [91]. A recent study has reported a thermoelectric figure of merit (ZT) up to 1.4 with graphene and C60 clusters synthesized by chemical vapor deposition (CVD) [92]. Another theoretical investigation revealed three peak ZT values of 2.0, 2.7 and 6.1 at 300 K, with a twisted bilayer graphene nanoribbon junction [93]. As shown in Figure4, the search for new thermoelectric materials is growing exponentially, but some categories are more attractive than others. Energies 2020, 13, 3606 8 of 32 Energies 2020, 13, x FOR PEER REVIEW 8 of 34

(a)

(b)

Figure 4. ((aa)) State State of of the the art thermoelement research research findings ﬁndings by by year; ( b) Development rate rate in in the thermoelementsthermoelements research from 1950 to 2017. Data from [94]. [94].

5. Applications Applications of of Thermoelectric Thermoelectric Generators Generators Thermoelectric applicationsapplications areare classified classified according according to theto twothe efftwoects effects characterizing characterizing the process, the process,namely thenamely Seebeck the eSeebeckffect, for effect, any application for any application that generates that generates electricity electricity with a temperature with a temperature difference, difference,and the Peltier and etheffect Peltier for any effect cooling for application any cooling powered application by an powered electric current. by an electric The latter current. case is The not latterexamined case inis not this examined article; only in this applications article; only that a generatepplications electricity that generate are presented. electricity are presented. To generate electricity from a TE module, it is necessary for there there to be be a a temperature temperature difference difference between its hot and cold surfaces. In In other other words, words, it it is is necessary necessary that that the the heat heat recovered recovered from the hot source scatters into into the the semiconductor semiconductor elements elements p p and and n n of of the the module, module, and and then then to to the the cold cold source, source, which isis usually usually the the environment. environment. TEG TEG applications applications can be can classified be classified into three into categories, three categories, depending dependingon the nature on ofthe the nature hot source: of the hot (i) radioisotopesource: (i) radioi heatsotope source, heat (ii) source, natural (ii) heat natural source, heat and source, (iii) waste and (iii)heat waste source. heat source.

5.1. Radioisotope Heat Source

Energies 2020, 13, 3606 9 of 32

Energies 2020, 13, x FOR PEER REVIEW 9 of 34 5.1. Radioisotope Heat Source A radioisotope thermoelectric generator (RTG) is a nuclear electric generator of simple design. A radioisotope thermoelectric generator (RTG) is a nuclear electric generator of simple design. It involves neither a process of fusion nor nuclear fission, which would require significant It involves neither a process of fusion nor nuclear fission, which would require significant constraints constraints on the system, but the natural decay of a radioactive atom, usually plutonium 238 in the on the system, but the natural decay of a radioactive atom, usually plutonium 238 in the form of form of plutonium dioxide 238PuO2. As they disintegrate, radioactive atoms release heat, some of plutonium dioxide 238PuO . As they disintegrate, radioactive atoms release heat, some of which is which is directly converted 2into electricity [95]. directly converted into electricity [95]. The first RTG was developed by Mound Laboratories in 1954 [96]. The heat source consisted of The first RTG was developed by Mound Laboratories in 1954 [96]. The heat source consisted of a a 1-cm diameter sphere containing 57 Ci (1.8 Wt) of 210Po inside a nickel-coated steel capsule, all in a 1-cm diameter sphere containing 57 Ci (1.8 Wt) of 210Po inside a nickel-coated steel capsule, all in Lucite container. With silver soldered chromel–constantan Thermocouples, the “thermal battery” a Lucite container. With silver soldered chromel–constantan Thermocouples, the “thermal battery” produced 1.8 mWe [97]. Three areas, namely the space domain, power supply devices in remote produced 1.8 mWe [97]. Three areas, namely the space domain, power supply devices in remote areas, areas, and the medical domain, have benefited from RTGs, although the last two areas have not and the medical domain, have benefited from RTGs, although the last two areas have not flourished flourished because of the risks involved in using radioisotopes. because of the risks involved in using radioisotopes. 5.1.1. Space Domain 5.1.1. Space Domain The first RTG launched into space by the United States of America was the SNAP 3B in 1961, The first RTG launched into space by the United States of America was the SNAP 3B in 1961, powered by 96 g of plutonium metal 238, aboard the Navy Transit 4A spacecraft [98]. In 2010, USA powered by 96 g of plutonium metal 238, aboard the Navy Transit 4A spacecraft [98]. In 2010, USA launched 41 RTGs on 26 space systems [99]. Among them were Galileo (launched in 1989 at Jupiter), launched 41 RTGs on 26 space systems [99]. Among them were Galileo (launched in 1989 at Jupiter), Ulysses (launched in 1990 as a solar orbital), Cassini (launched in 1997 at Saturn), New Horizons Ulysses (launched in 1990 as a solar orbital), Cassini (launched in 1997 at Saturn), New Horizons (launched in 2006 to fly over Pluto 2015) and the Curiosity robot from the Mars Science Laboratory (launched in 2006 to fly over Pluto 2015) and the Curiosity robot from the Mars Science Laboratory (installed on Mars in 2012). Systems for nuclear auxiliary power units (SNAPs) were used for probes (installed on Mars in 2012). Systems for nuclear auxiliary power units (SNAPs) were used for probes that travelled far from the Sun, conditions that make solar panels impractical [100]. that travelled far from the Sun, conditions that make solar panels impractical [100]. The RTGs used in the US space program initially included SiGe TE materials installed in the The RTGs used in the US space program initially included SiGe TE materials installed in the General General Purpose Heat Source–Radioisotope Thermoelectric Generator (GPHS-RTG) GPHS-RTG, Purpose Heat Source–Radioisotope Thermoelectric Generator (GPHS-RTG) GPHS-RTG, later succeeded later succeeded by the lead telluride alloys, or TAGS, used in multi-mission RTGs (MMRTGs), by the lead telluride alloys, or TAGS, used in multi-mission RTGs (MMRTGs), shown in Figure5. shown in Figure 5. This MMRTG was developed, under the program called enhanced MMRTG or This MMRTG was developed, under the program called enhanced MMRTG or eMMRTG [101], with eMMRTG [101], with the use of new skutterudite thermoelectric materials in order to achieve higher the use of new skutterudite thermoelectric materials in order to achieve higher efficiency and lower efficiency and lower degradation rates, which are important for long-term missions to the outer degradation rates, which are important for long-term missions to the outer planets [102]. planets [102].

Figure 5. Cutting view of an Multi-Mission Radioisotope Thermoelectric Generator MMRTG [103]. Figure 5. Cutting view of an Multi-Mission Radioisotope Thermoelectric Generator MMRTG [103].

5.1.2. Power Supply Devices in Remote Areas

Energies 2020, 13, 3606 10 of 32 Energies 2020, 13, x FOR PEER REVIEW 10 of 34 Energies 2020, 13, x FOR PEER REVIEW 10 of 34 5.1.2.One Power of Supplythe first Devices terrestrial in Remote uses Areasof RTGs was in 1966 by the U.S. Navy for powering One of the first terrestrial uses of RTGs was in 1966 by the U.S. Navy for powering environmentalOne of the instrumentation ﬁrst terrestrial uses at ofFairway RTGs wasRock, in a 1966 small by uninhabited the U.S. Navy island for powering in Alaska. environmental RTGs were environmental instrumentation at Fairway Rock, a small uninhabited island in Alaska. RTGs were instrumentationused at this site atuntil Fairway 1995 Rock, [104]. a smallThese uninhabitedsystems were island develop in Alaska.ed for RTGs the weresupply used of atpower this site to used at this site until 1995 [104]. These systems were developed for the supply of power to untilequipment 1995 [104requiring]. These a systems stable wereand developedreliable power for the source, supply ofover power several to equipment years and requiring without a equipment requiring a stable and reliable power source, over several years and without stablemaintenance. and reliable Examples power of source, these overwould several be powe yearsr andsupplies without for maintenance.systems located Examples in isolated of these or maintenance. Examples of these would be power supplies for systems located in isolated or wouldinaccessible be power environments, supplies for like systems lighthouses, located in navi isolatedgation or inaccessiblebeacons and environments, weather stations. like lighthouses, Similarly, inaccessible environments, like lighthouses, navigation beacons and weather stations. Similarly, navigationbetween 1960 beacons and and1980, weather the Soviet stations. Union Similarly, built many between unmanned 1960 and lighthouses 1980, the Soviet and Unionnavigation built between 1960 and 1980, the Soviet Union built many unmanned lighthouses and navigation manybeacons, unmanned equipped lighthouses with about and 1000 navigation RTGs beacons, (Figure equipped6) [105]. withAll Russian about 1000 RTGs RTGs have (Figure long6)[ since105]. beacons, equipped with about 1000 RTGs (Figure 6) [105]. All Russian RTGs have long since Allexhausted Russian their RTGs 10-year have longlifespan, since and exhausted require theirextrem 10-yeare dismantling lifespan, measures and require due extreme to their dismantling status as a exhausted their 10-year lifespan, and require extreme dismantling measures due to their status as a measurespotentially due dangerous to their statussource as of a potentiallyradioactivity, dangerous and the sourcerisk they of radioactivity,pose of being and used the in risk terrorist they poseacts potentially dangerous source of radioactivity, and the risk they pose of being used in terrorist acts of[106]. being Obviously, used in terrorist all research acts [and106 ].development Obviously, allin researchthis field and has developmentbeen stopped in because this ﬁeld of hasthe beenrisks [106]. Obviously, all research and development in this field has been stopped because of the risks stoppedalready mentioned because of [107]. the risks already mentioned [107]. already mentioned [107].

Figure 6. Radioisotope Thermoelectric Generator Generator (RTG (RTG)) used to supply power to lighthouses and Figure 6. Radioisotope Thermoelectric Generator (RTG) used to supply power to lighthouses and navigation beacons [[106].106]. navigation beacons [106]. 5.1.3. Medical Domain 5.1.3. Medical Domain In 1966, small plutonium cells (very small RTGs fed with Pu238) were used in implantedimplanted pacemakersIn 1966, to small ensure plutonium a very long cells battery (very life, small as shownRTGs infed Figure with7 7Pu238)[ [108].108]. Inwere 2004, used about in 90implanted of them pacemakers to ensure a very long battery life, as shown in Figure 7 [108]. In 2004, about 90 of them were stillstill in in use. use. Many Many companies companies have have manufactured manufactured nuclear-powered nuclear-powered pacemakers, pacemakers, including including ARCO were still in use. Many companies have manufactured nuclear-powered pacemakers, including (Perma-grain),ARCO (Perma-grain), Medtronic Medtronic (Laurens-Alcatel), (Laurens-Alcatel), Gulf General Gulf Atomic,General CordisAtomic, (Telektronic, Cordis (Telektronic, Accuﬃx), ARCO (Perma-grain), Medtronic (Laurens-Alcatel), Gulf General Atomic, Cordis (Telektronic, AmericanAccuffix), Optical,American Technologie Optical, Technologie Biocontrol (Coratomic) Biocontrol and(Coratomic) Medical Devices,and Medical Inc (MDI) Devices, [109 ].Inc After (MDI) the Accuffix), American Optical, Technologie Biocontrol (Coratomic) and Medical Devices, Inc (MDI) development[109]. After the of development lithium batteries, of lithium the market batteries, for nuclear the market batteries for nuclear driedup batteries [110]. dried up [110]. [109]. After the development of lithium batteries, the market for nuclear batteries dried up [110].

Figure 7. Left pacemaker and right RTG battery [108]. Figure 7. Left pacemaker and right RTG battery [[108].108].

5.2. Natural Heat Source 5.2. Natural Heat Source

Energies 2020, 13, 3606 11 of 32

5.2. Natural Heat Source Energies 2020, 13, x FOR PEER REVIEW 11 of 34 5.2.1. Natural Gas and Biomass 5.2.1. Natural Gas and Biomass Pouillet, in 1840, employed the Seebeck effect in making a thermoelectric cell with a welded pair of bismuthPouillet, and in copper. 1840, employed The two solderings the Seebeck were effect immersed in making in two a thermoelectric vessels, one containing cell with melting a welded ice andpair theof bismuth other hot and water copper. [111]. ThisThe apparatustwo solderings supplying were aimmersed constant source in two of vessels, dynamic one electricity containing was usedmelting by theice and author the to other investigate hot water the general[111]. This laws apparatus of currents supplying [112]. a constant source of dynamic electricityAt the was same used time by several the author prototypes to inve ofstigate thermoelectric the general batteries laws wereof currents built and [112]. even commercialized, withAt diff erentthe same sizes andtime materials several forprototypes different purposesof thermoelectric [113]. For instance,batteries thewere Oersted built andand Fourier even batterycommercialized, they designed with fordifferent their investigations sizes and materials [114], the for Ruhmkor differentff Thermopilepurposes [113]. (1860) For powered instance, by gasthe andOersted cooled and with Fourier water battery [115], and they the designed gigantic for Clamond their investigations battery (1879), [114], which the was Ruhmkorff the first thermoelectric Thermopile battery(1860) powered powered by with gas coaland orcooled wood, with and water which [115 could], and have the gigantic been used Clamond in industry, battery with (1879), a height which of 2.50was mthe and first 1 mthermoelectric in diameter. Itsbattery maximum powered power with was co 192al or Watts, wood, at and 54 Volts which and could 3.5 Amperes have been [116 used]. It in is worthyindustry, of with note thata height the most of 2.50 remarkable m and 1achievement m in diameter. was Its Melloni’s maximum Thermo-Multiplier, power was 192 builtWatts, in at 1830, 54 whichVolts and was an3.5 instrumentAmperes [116]. for making It is worthy very small of amountsnote that ofthe sensitive most remarkable heat. The battery achievement consisted was of aboutMelloni’s 10 Bismuth-antimony Thermo-Multiplier, pairs built that in 1830, were which associated was an with instrument a Nobili for galvanometer. making very This small instrument amounts wasof sensitive so sensitive heat. that The the battery galvanometer consisted needle of about was 10 deflected Bismuth-antimony by the presence pairs of that the naturalwere associated heat of a personwith a placedNobili 10galvanometer. m from the battery This instrument [117]. was so sensitive that the galvanometer needle was deflectedNowadays, by the presence thermopiles of the or natural TEG thermoelectric heat of a person generators placed 10 are m from designed the battery to supply [117]. energy to autonomousNowadays, sensors, thermopiles installed or in TEG remote thermoelectric locations subject generators to severe are environmentaldesigned to supply conditions, energy i.e., to veryautonomous low-temperature sensors, andinstalled difficult-to-access in remote locations locations, subject where to conventional severe environmental renewable conditions, energy sources, i.e., suchvery aslow-temperature solar and wind energy,and difficult-to-access are not regularly lo available.cations, where Heat isconventional usually supplied renewable by a flameless energy catalyticsources, such burner as [118solar]. Aand few wind manufactures energy, are of not thermoelectric regularly available. generators Heat powered is usually by naturalsupplied gas by are a installedflameless in catalytic more than burner 55 countries. [118]. A For few instance, manufact Genthermures of manufacturesthermoelectric TEGs generators with powers powered ranging by fromnatural 15 gas to 550are W.installed These in generators more than are 55 mainly countries. used For on instance, offshore Gentherm platforms, manufactures along pipelines, TEGs at highwith altitudespowers ranging or near gasfrom wells 15 to (Figure 550 W.8)[ These119]. Anothergenerators example are mainly is Farwest used Corrosionon offshore Control, platforms, a company along thatpipelines, manufactures at high altitudes and installs or near TEGs gas forwells cathodic (Figure protection 8) [119]. Another against example pipe corrosion, is Farwest and Corrosion that has installedControl, morea company than 15,000 that generatorsmanufactures in 51 and countries installs [120 TEGs]. for cathodic protection against pipe corrosion, and that has installed more than 15,000 generators in 51 countries [120].

Figure 8. Gentherm Gas TEG [[119].119]. Several products designed for public use have been marketed. One such was the thermoelectric Several products designed for public use have been marketed. One such was the thermoelectric candle radio (1990), which uses the heat from candles to power a radio via a FeSi2 TE module [121]. candle radio (1990), which uses the heat from candles to power a radio via a FeSi2 TE module [121]. These applications have become obsolete with the emergence of other more practical technologies, These applications have become obsolete with the emergence of other more practical technologies, but despite that, more speciﬁc applications are still emerging, such as the CampStove shown in Figure9. but despite that, more specific applications are still emerging, such as the CampStove shown in This appliance, designed for camping in general, burns wood to produce 2 W of 0.4 A and 5 V power Figure 9. This appliance, designed for camping in general, burns wood to produce 2 W of 0.4 A and 5 using a thermoelectric generator to which the connection of the electrical devices is made via a USB V power using a thermoelectric generator to which the connection of the electrical devices is made port [122]. via a USB port [122].

Energies 2020, 13, x FOR PEER REVIEW 12 of 34 Energies 2020, 13, 3606 12 of 32 Energies 2020, 13, x FOR PEER REVIEW 12 of 34

Figure 9. Picture of a CampStove [122]. Figure 9. Picture of a CampStove [122]. Figure 9. Picture of a CampStove [122]. 5.2.2. Human Body 5.2.2. Human Body As the heatheat ofof thethe humanhuman bodybody is is natural natural and and stable, stable, it it could could be be used used to to supply supply some some electricity electricity in veryin veryAs specific specificthe heat applications, applications, of the human such such body as medicalas ismedical natural ones ones and [123 [123,124]. stab,124le,]. Theit couldThe human human be used body body to releases supply releases around some around electricity 100 100 W W of heatofin veryheat at rest,atspecific rest, and and applications, 525 525 W W during during such physical physical as medical eff orteffort ones [125 [125].]. [123,124]. The human body releases around 100 W of heatSeveral at rest, investigations and 525 W during have beenphysical conducted effort [125]. into wearable thermoelectric generators (WTEGs) sinceSeveral 20012001 [126[126], investigations], with with the the aim haveaim of substituting ofbeen substituting conducted lithium-ion lithium-ion into wearable batteries batteries thermoelectric [127] as[127] a power as generatorsa sourcepowerfor source (WTEGs) portable for devices,portablesince 2001 givendevices, [126], that givenwith the global thethat aim the market ofglobal substituting for market portable folithium-ion technologiesr portable technologiesbatteries is growing [127] is rapidly growingas a andpower rapidly is expectedsource and for tois exceedexpectedportable USD devices,to 34exceed billion given USD by that 202034 billion the and global USD by 2020 78 market billion and fo byUSDr 2021portable 78 [128billion ].technologies WTEGs by 2021 are [128]. classifiedis growing WTEGs according rapidly are classified toand their is rigidexpectedaccording or flexible to to exceed their (extensible rigid USD or 34 orflexible billion not) [ 129(extensibleby ]2020 architectures and or USD not) in 78[129] 2D billion or architectures 3D by configurations 2021 [128].in 2D WTEGsor [130 3D], orconfigurations are according classified to their[130],according TE or componentaccording to their rigid to materials, their or flexibleTE component which (extensible are inorganic, material or nos, organict) which [129] arearchitectures or hybrid inorganic, [131 in or]. 2Dganic or or3D hybrid configurations [131]. [130],Leonov or according and Vullers to their [[124]124 TE] component publishedpublished anmaterial interestinginterestings, which review are inorganic, on WTEGs,WTEGs, organic focusing or hybrid on rigid- [131]. and flexible-typeflexible-typeLeonov thermoelectricand Vullers [124] generators. published They an interestingconc concludedluded thatreview the on wearable WTEGs, thermoelectric focusing on rigidgenerator and styleflexible-type waswas mature,mature, thermoelectric and and that that the generators.the major major concern concern They was conc was toluded improveto improve that the the ethe ffiwearableciency efficiency of thermoelectric the of generator the generator andgenerator make and itstylemake thinner wasit thinner andmature, more and and flexible.more that flexible. the These major These authors concern authors conducted was conducted to improve extensive extensive the research efficiency research on rigidof theon substrate rigidgenerator substrate TEGs. and TheyTEGs.make developed itThey thinner developed diandfferent more WTEGdifferent flexible. products WTEGThese that authorsproducts used bodyconducted that heat, used such extensive body as the heat, wirelessresearch such electrocardiography on as rigid the substratewireless systemelectrocardiographyTEGs. They integrated developed into system an different offi integratedce shirt. WTEG This into product anproducts office was shirt. that powered Thisused product bybody 17 smallwasheat, powered TEsuch modules as by the 17 integrated smallwireless TE intomoduleselectrocardiography the front integrated side of intosystem a shirt, the asintegratedfront shown side in intoof Figure a anshirt, office 10 as. They shownshirt. converted This in Figure product the 10. body’swas They powered naturalconverted by heat 17 the flowsmall body’s into TE 0.8–3naturalmodules mW heat integrated of electricalflow into into energy, 0.8–3 the front mW depending siofde electrical of ona shirt, the en physical asergy, shown depending activity in Figure of theon 10. personthe They physical [converted132]. activity the body’sof the personnatural [132].heat flow into 0.8–3 mW of electrical energy, depending on the physical activity of the person [132].

Figure 10. WearableWearable Thermoelectric Generator (WTEG)(WTEG) integrated into a shirt [[132].132]. Figure 10. Wearable Thermoelectric Generator (WTEG) integrated into a shirt [132]. One of the leading companies in this ﬁeld is IMEC (Belgium), which has been working on One of the leading companies in this field is IMEC (Belgium), which has been working on thermoelectric generation by people since the 2000s, with a view to power electronic health care thermoelectricOne of the generation leading companies by people insince this thefield 2000 is s,IMEC with (Belgium),a view to whichpower haselectronic been workinghealth care on systems. IMEC and the Holst Centre have developed several wireless sensors, such as the Body-powered systems.thermoelectric IMEC generation and the byHolst people Centre since have the 2000developeds, with severala view towireless power sensors,electronic such health as carethe Electroencephalogram Acquisition System, which produces 2–2.5 mW of power and is worn as a systems.Body-powered IMEC Electroencephalogram and the Holst Centre Acquisition have developed System, whichseveral produces wireless 2–2.5 sensors, mW ofsuch power as andthe Body-powered Electroencephalogram Acquisition System, which produces 2–2.5 mW of power and

Energies 2020, 13, 3606 13 of 32

Energies 2020, 13, x FOR PEER REVIEW 13 of 34 headband, shown in Figure 11a [133]. They have also developed a wireless pulse oximeter (2006), is worn as a headband, shown in Figure 11a [133]. They have also developed a wireless pulse powered entirely by a TEG-style watch using commercial Bi2Te3 thermopiles, shown in Figure 11b, andoximeter in which (2006), the generatorpowered developsentirely by around a TEG-style 89 µW of watch power using [134]. commercial Bi2Te3 thermopiles, shown in Figure 11b, and in which the generator develops around 89 µW of power [134].

(a)

(b)

Figure 11. ((aa)) TEG TEG supplies supplies power power to to an an EEG EEG system system mounted mounted on onan anexpandable expandable headband headband [133]; [133 (b];) (Wirelessb) Wireless pulse pulse oximeter oximeter [134]. [134 ].

The firstfirst thermoelectric wristwatch powered by converting body heat into electrical power was marketed by Seiko and Citizen, and dates backback toto 19991999 [[135,136].135,136]. The The Seiko Seiko watch watch [Figure (Figure 12a]12a) produced 22 µµWW of electrical power, andand anan open-circuitopen-circuit voltage of 300 mV with eefficiencyfficiency of about 0.1% [[137].137]. AnotherAnother exampleexample is is the the Dyson Dyson TE TE bracelet bracelet (2012), (2012), shown shown in Figure in Figure 12b, which,12b, which, using using body heat,body chargedheat, charged a battery a battery integrated integrated into it for into charging it for charging a mobile phonea mobile or anyphone other or mobileany other device. mobile device.Because of the drawbacks of rigid modules, i.e., the high thermal resistance between skin and the TEG, flexible modules are more suitable for power generation from body heat, as they can be adapted to the shape of the body, thus increasing the useful surface area for heat capture and reducing thermal contact resistance [139]. Francioso et al. [123] developed a flexible and wearable micro thermoelectric generator, composed of an array of 100 thin film thermocouples of Sb2Te3 and Bi2Te3, designed to power very low-consumption electronical ambient assisted living (AAL) applications. The best result obtained was 430 mV in open circuit, and an electrical output power up to 32 nW at 40 ◦C. Kim et al. [140] manufactured a flexible fabric-shaped(a) TEG, with 3D printing technology, composed of 20 thermocouples and with a thickness of 0.5 mm, as shown in Figure 13a. The TEG, when applied to a human body, generated an electrical power of 25 mV at an ambient temperature of 5 ◦C. A new approach was presented by Suarez et al. [141], using standard bulk legs interconnected to a stretchable low-resistivity eutectic alloy of gallium and indium (EGaIn), all in a flexible elastomer package, as

Energies 2020, 13, x FOR PEER REVIEW 13 of 34 is worn as a headband, shown in Figure 11a [133]. They have also developed a wireless pulse oximeter (2006), powered entirely by a TEG-style watch using commercial Bi2Te3 thermopiles, shown in Figure 11b, and in which the generator develops around 89 µW of power [134].

(a)

Energies 2020, 13, x FOR PEER REVIEW 14 of 34

(b) Energies 2020, 13, 3606 14 of 32 Figure 11. (a) TEG supplies power to an EEG system mounted on an expandable headband [133]; (b) Wireless pulse oximeter [134]. shown in Figure 13b. The authors reported a ﬁgure of merit (ZT) of 0.35, which they claimed to be better thanThe that first of any thermoelectric other similar wristwatch device reported powered in the by open converting literature. body Zadan heat et into al. [142 electrical] introduced power a was soft and stretchable thermoelectric generator (TEG) with the ability to expand, to explore the integration marketed by Seiko and Citizen, and dates back to 1999 [135,136]. The Seiko watch [Figure 12a] ofproduced this TEG 22 into µW wearable of electrical technologies. power, and All an investigations open-circuit voltage conducted of 300 so mV far havewith suggestedefficiency of that about this (b) option0.1% [137]. is only Another viable atexample moderate is temperatures,the Dyson TE i.e., bracelet indoors (2012), in particular, shown andin Figure this limits 12b, itswhich, application using together with the high cost of TE modules [143]. body heat, chargedFigure a battery 12. (a) Seikointegrated Thermic, into wristwatch it for charging [137]; (b )a Dyson mobile bracelet phone [138]. or any other mobile device. Because of the drawbacks of rigid modules, i.e., the high thermal resistance between skin and the TEG, flexible modules are more suitable for power generation from body heat, as they can be adapted to the shape of the body, thus increasing the useful surface area for heat capture and reducing thermal contact resistance [139]. Francioso et al. [123] developed a flexible and wearable micro thermoelectric generator, composed of an array of 100 thin film thermocouples of Sb2Te3 and BiEnergies2Te3, 2020designed, 13, x FOR to PEER power REVIEW very low-consumption electronical ambient assisted living (AAL)14 of 34 applications. The best result obtained was 430 (a)mV in open circuit, and an electrical output power up to 32 nW at 40 °C. Kim et al. [140] manufactured a flexible fabric-shaped TEG, with 3D printing technology, composed of 20 thermocouples and with a thickness of 0.5 mm, as shown in Figure 13a. The TEG, when applied to a human body, generated an electrical power of 25 mV at an ambient temperature of 5 °C. A new approach was presented by Suarez et al. [141], using standard bulk legs interconnected to a stretchable low-resistivity eutectic alloy of gallium and indium (EGaIn), all in a flexible elastomer package, as shown in Figure 13b. The authors reported a figure of merit (ZT) of 0.35, which they claimed to be better than that of any other similar device reported in the open literature. Zadan et al. [142] introduced a soft(b) and stretchable thermoelectric generator (TEG) with the ability to expand, to explore the integration of this TEG into wearable technologies. All Figure 12. a b investigations Figure 12. (a) Seiko Thermic, wristwatch [137]; [137]; (b) Dyson bracelet [[138].138].

Because of the drawbacks of rigid modules, i.e., the high thermal resistance between skin and the TEG, flexible modules are more suitable for power generation from body heat, as they can be adapted to the shape of the body, thus increasing the useful surface area for heat capture and reducing thermal contact resistance [139]. Francioso et al. [123] developed a flexible and wearable micro thermoelectric generator, composed of an array of 100 thin film thermocouples of Sb2Te3 and Bi2Te3, designed to power very low-consumption electronical ambient assisted living (AAL) applications. The best result obtained was 430 mV in open circuit, and an electrical output power up to 32 nW at 40 °C. Kim et al. [140] manufactured a flexible fabric-shaped TEG, with 3D printing technology, composed of 20 thermocouples and with a thickness of 0.5 mm, as shown in Figure 13a. The TEG, when applied to a human body, generated an electrical power of 25 mV at an ambient temperature of 5 °C. A new approach was presented(a) by Suarez et al. [141], using standard bulk legs interconnected to a stretchable low-resistivity eutectic alloy of gallium and indium (EGaIn), all in a flexible elastomer package, as shown in Figure 13b. The authors reported a figure of merit (ZT) of 0.35, which they claimed to be better than that of any other similar device reported in the open literature. Zadan et al. [142] introduced a soft and stretchable thermoelectric generator (TEG) with the ability to expand, to explore the integration of this TEG into wearable technologies. All investigations conducted so far have suggested that this option is only viable at moderate temperatures, i.e., indoors in particular, and this limits its application together with the high cost of TE modules [143].

(b)

Figure 13. (a) Flexible TEG fabricated using dispenser printing technology [144]. (b) Flexible device and test conﬁguration: (A) ﬂexible device tested, (B) circuit voltage open at room temperature [141].

(a)

Energies 2020, 13, 3606 15 of 32

Soleimani et al. [145] published a review on recent developments in inorganic, organic and hybrid wearable TEGs. The authors concluded that inorganic TE materials remain favorable due to their high ZT (~1), but unfavorable due to their rarity, toxicity and impractical rigidity. Organic TE materials have a high flexibility and contain non-toxic elements, but their weaknesses are their low stability in air and the complexity of their synthesis process. They reported that hybrid TE materials are the solution to the rigidity of inorganic TE materials and the low efficiency of organic TE materials. These hybrid TE materials are suitable for portable TEGs. Jiang et al. [146] presented a review focused on the recent developments of TE materials, concerning film- and fiber-based materials for flexible, wearable applications. They concluded that these applications will eventually become a reality with the development of preparation technology for film or fiber legs, and the emergence of human thermoregulatory models for the designing of wearable devices and their integration with other wearable renewable energy conversion devices.

5.2.3. Sun Source A solar thermoelectric generator (STEG) is a system designed to recover heat from solar radiation and convert it into electricity using a thermoelectric generator (TEG). It is becoming a technological alternative, and is competing with the dominant solar photovoltaic systems despite its low conversion efficiency compared to photovoltaic technology [147]. STEGs are classified according to the type of optical sensors used, namely, an optical concentration system or not. Optical concentration sensors are usually cylindrical lenses, Fresnel lenses, parabolic mirrors, flat mirrors or parabolic concentrators. Non-concentrated solutions are rather limited to flat plate collectors evacuated or not evacuated, and vacuum tubes [148]. Karthick et al. [149], in a recent review, reported that the use of optical concentrators, combined with heat pipe tubes, improves the efficiency of solar thermoelectric generators (STEG). The first investigation into thermoelectric solar energy dates back to the end of the 19th century, with Weston’s patent in 1888 [150,151], which combined a thermopile (TEG), with a mirror or lens to focus solar radiation on hot junctions, and a storage battery. In 1910, Sun Electric Generator Company [152] published claims concerning the functioning of a thermoelectric solar generator. The first experimental data on an STEG device were published by Coblentz in 1922 [153], describing an efficiency of less than 0.01%. More progress was reported in 1954 concerning a STEG device designed by Telkes [154], who demonstrated an efficiency of the optical concentrators in a flat-plate solar energy collector of 0.63% and 3.35%, at 1 x and 50 x suns (1 sun = 1000 W/m2), respectively. ZnSb-type alloys in combination with a negative Bi-alloy were used. The most significant investigations into solar thermoelectric generators (STEGs) are summarized in the following. He et al. [155] carried out a theoretical and experimental study on the integration of thermoelectric modules into solar vacuum tube heaters (SHP-TE), as illustrated in Figure 14. Their experimental data showed an electrical efficiency of 1%, which is slightly lower than a system with an organic Rankine cycle, but according to the authors the SHP-TE system is simple and has no moving components, and its units are easy to replace. Following the same principle, another creative investigation was done with the use of a parabolic solar concentrator [156,157]. Kraemer et al. [158] used nanostructured thermoelectric materials to develop the solar flat panel thermoelectric generators shown in Figure 15. These TEs achieved a maximum efficiency of 4.6% 2 under an irradiation of 1 kW/m− . The efficiency was seven to eight times higher than the best value previously reported for a flat panel display. Amatya and Ram [159] combined a commercial Bi2Te3 module with a parabolic concentrator (solar concentration of 66 x suns. A system efficiency of 3% was measured and an output power of 1.8 W was achieved. Rehman et al. [160] proposed a novel collector design for a solar concentrated thermoelectric generator. The system had an electrical efficiency of 1.45% and a maximum optical efficiency of 93.61%. Li et al. [161] evaluated a prototype, consisting of a solar concentration thermoelectric generator (CTG) with a Fresnel lens (Figure 16). Their results showed that the highest possible CTG efficiency Energies 2020, 13, x FOR PEER REVIEW 16 of 34

Energies 2020, 13, 3606 16 of 32

could reach 9.8%, 13.5% and 14.1%, for Bi2Te3, skutterudite and silver antimony lead telluride (LAST) EnergiesFigure 2020 , 14.13, xSchematic FOR PEER REVIEWdiagram of an integrated Solar Heat-Pipe/Thermoelectric (SHP-TE) system16 of 34 alloys, respectively. [155].

Kraemer et al. [158] used nanostructured thermoelectric materials to develop the solar flat panel thermoelectric generators shown in Figure 15. These TEs achieved a maximum efficiency of 4.6% under an irradiation of 1 kW/m−2. The efficiency was seven to eight times higher than the best value previously reported for a flat panel display. Amatya and Ram [159] combined a commercial Bi2Te3 module with a parabolic concentrator (solar concentration of 66 x suns . A system efficiency of 3% was measured and an output power of 1.8 W was achieved. Rehman et al. [160] proposed a novel collector design for a solar concentrated thermoelectric generator. The system had an electrical efficiency of 1.45% and a maximum optical efficiency of 93.61%. Li et al. [161] evaluated a prototype, consisting of a solar concentration thermoelectric generator (CTG) with a Fresnel lens (Figure 16). Their results showed that the highest possible CTG efficiency could reach 9.8%, 13.5% and 14.1%, for Bi2Te 3, skutterudite and silver Figure 14. Schematic diagram of an integrated Solar Heat-Pipe/Thermoelectric (SHP-TE) system [155]. antimony lead telluride (LAST) alloys, respectively. Figure 14. Schematic diagram of an integrated Solar Heat-Pipe/Thermoelectric (SHP-TE) system [155].

Energiesthermoelectric 2020, 13, x FOR generator. PEER REVIEW The system had an electrical efficiency of 1.45% and a maximum17 ofoptical 34 efficiencyFigure of 15.15. 93.61%.IllustrationIllustration Li of ofet a a STEG al.STEG [161] cell cell consisting evaluatedconsisting of aofa pair aprototype, pa ofir thermoelectric of thermoelectric consisting elements elementsof a type solar ptype and concentration p n and [158 n]. thermoelectric[158]. generator (CTG) with a Fresnel lens (Figure 16). Their results showed that the highest possible CTG efficiency could reach 9.8%, 13.5% and 14.1%, for Bi2Te3, skutterudite and silver antimony lead telluride (LAST) alloys, respectively.

Figure 15. Illustration of a STEG cell consisting of a pair of thermoelectric elements type p and n Figure 16. Concentrating Solar Thermoelectric Generators (CTG): (a) An experimental prototype of the [158]. Concentrating Solar Thermoelectric Generator System; (b) Details of the CTG unit [149].

FigurePhotovoltaic 16. Concentrating and thermoelectric Solar Thermoelec systemstric are Generators the only processes(CTG): (a) thatAn experimental directly convert prototype solar energy of into electricalthe Concentrating energy [162 Solar,163 Thermo]. Someelectric investigations Generator were System; conducted (b) Details on of hybrid the CTG photovoltaic–thermoelectric unit [149]. systems [164] and concentrated photovoltaic (CPV)–thermoelectric systems [165]. Another hybrid system consistsPhotovoltaic of the direct and couplingthermoelectric of a solar systems water heaterare the with only a thermoelectricprocesses that module, directly in orderconvert to improvesolar energythe overall into electrical performance energy of [162,163]. the system Some producing investigations heat and were electricity conducted simultaneously on hybrid photovoltaic– [166]. However, thermoelectrictheir combination systems is complex [164] and because concentrated of their opposing photovoltaic characteristics, (CPV)–thermoelectric and effective systems integration [165]. of the Anothertwo systems hybrid is system essential consists [167]. of Sripadmanabhan the direct coupling Indira of et a al.solar [168 water] investigated heater with different a thermoelectric configurations module,of the in hybrid order systemto improve integrating the overall photovoltaic performanc concentratorse of the system (CPV) producing and thermoelectric heat and electricity generators simultaneously [166]. However, their combination is complex because of their opposing characteristics, and effective integration of the two systems is essential [167]. Sripadmanabhan Indira et al. [168] investigated different configurations of the hybrid system integrating photovoltaic concentrators (CPV) and thermoelectric generators (TEG), and outlined recommendations for future research. The authors reported that the integrated CPV-TEG-based solar thermal systems achieve higher electrical and thermal performances than those of non-concentrated PV-TEG systems. Li et al. [169] compared a hybrid photovoltaic–thermoelectric (PV-TE) system employing a micro-channel heat pipe array together with PV electricity generation. The results showed that the electrical efficiencies of the hybrid PV-TE system were about 14% higher than those obtained with the PV system. Mizoshiri et al. [170] built a hybrid module composed of a thin-film thermoelectric module and a photovoltaic module. This hybrid module filtered light through an infrared filter (hot mirror), allowing only the light that contributed to photovoltaic conversion to pass through. At the same time, the reflected light was focused on the warm side of the thermoelectric module using a lens. The total no-load voltage of the hybrid thermo-photovoltaic generator showed an increase of 1.3% when compared with the photovoltaic module alone. As regards TPV/TE hybrid systems, thermophotovoltaic cells (TPV) are capable of converting infrared radiation into electricity. They consist of a heat source, an emitter, a filter and photovoltaic (PV) cells [171]. Unlike photovoltaic solar panels, TPV cells are illuminated by radiant combustion sources. Given that the radiant power density of these sources can be much higher than that of the sun, the electrical power density of TPV cells is much higher than that of solar cells, with an efficiency of 24.5% [172]. As of yet, few studies have been conducted on integrated TPV/TE systems. Qiu and Hayden [173] reported that the efficiency of an integrated system with TPV GaSb cells and TEG was superior to that of individual TPV and TE. For this reason, the TPV/TE hybrid system is an interesting alternative system, and further research is required in the future [174]. The major concern with hybrid systems is to achieve optimal hybridization. This means ensuring that the sum of the maximum powers produced separately by the PV and TE systems equals the power produced by the hybrid system [175].

5.3. Waste Heat Source

Energies 2020, 13, 3606 17 of 32

(TEG), and outlined recommendations for future research. The authors reported that the integrated CPV-TEG-based solar thermal systems achieve higher electrical and thermal performances than those of non-concentrated PV-TEG systems. Li et al. [169] compared a hybrid photovoltaic–thermoelectric (PV-TE) system employing a micro-channel heat pipe array together with PV electricity generation. The results showed that the electrical efficiencies of the hybrid PV-TE system were about 14% higher than those obtained with the PV system. Mizoshiri et al. [170] built a hybrid module composed of a thin-film thermoelectric module and a photovoltaic module. This hybrid module filtered light through an infrared filter (hot mirror), allowing only the light that contributed to photovoltaic conversion to pass through. At the same time, the reflected light was focused on the warm side of the thermoelectric module using a lens. The total no-load voltage of the hybrid thermo-photovoltaic generator showed an increase of 1.3% when compared with the photovoltaic module alone. As regards TPV/TE hybrid systems, thermophotovoltaic cells (TPV) are capable of converting infrared radiation into electricity. They consist of a heat source, an emitter, a filter and photovoltaic (PV) cells [171]. Unlike photovoltaic solar panels, TPV cells are illuminated by radiant combustion sources. Given that the radiant power density of these sources can be much higher than that of the sun, the electrical power density of TPV cells is much higher than that of solar cells, with an efficiency of 24.5% [172]. As of yet, few studies have been conducted on integrated TPV/TE systems. Qiu and Hayden [173] reported that the efficiency of an integrated system with TPV GaSb cells and TEG was superior to that of individual TPV and TE. For this reason, the TPV/TE hybrid system is an interesting alternative system, and further research is required in the future [174]. The major concern with hybrid systems is to achieve optimal hybridization. This means ensuring that the sum of the maximum powers produced separately by the PV and TE systems equals the power produced by the hybrid system [175].

5.3. Waste Heat Source A huge amount of low-grade waste heat is released into the environment, without any attempt at heat recovery [43]. Over the last three decades, much effort has been made to improve the efficiency of thermoelectric technology used in heat recovery applications [176]. This is facilitated by the fact that TE technology could be easily adapted to the physical parameters, such as the temperature, pressure, and heat transfer fluid, of a given heat recovery application. Waste heat recovery using thermoelectric technology can be divided into two main groups, as follows.

5.3.1. Waste Heat Recovery from Industry and Homes Dai et al. [177] reported that, in United States, 33% of industrial manufacturing energy is released directly into the atmosphere or into cooling systems as waste heat, and this amount of heat could be used to produce 0.9 TWh to 2.8 TWh of electricity per year, if thermoelectric materials with average ZT values ranging from 1 to 2 were available. Hence, there is a need for technical and economic studies that will develop the feasibility of large-scale applications that, in the medium and long term, would make this a competitive source of clean energy [178]. Zou et al. [179] demonstrated that municipal wastewater can be used to produce electricity using a thermoelectric generator (TEG). Their theoretical study, performed for the Christiansburg Wastewater Treatment Plant, estimated energy generation of 1094 to 70,986 kWh per year, with a saving of USD 163 to USD 6076. In another investigation that used an air-cooling heat recovery device with 120-mm square pipes, the wastewater recovery eﬃciency was 1.28%, and the amortization period for the equipment extended to 8 years [180]. Araiz et al. [181] carried out a techno-economic study into the thermoelectric recovery of hot gases from a stone wool manufacturing plant. They reported a maximum net power production of 45 kW, and a Levelized Cost of Electricity at about 0.15 EUR/kWh, which demonstrated the feasibility of the system. Mirhosseini et al. [182] performed a similar investigation, using an arc-shaped absorber designed for the thermoelectric heat recovery of waste heat from a rotary cement kiln. The economic evaluation showed that the dominant parameter in the system cost is the heat sink. Energies 2020, 13, 3606 18 of 32

It is worth noting that heat recovery depends strongly on the ambient temperature. In hot regions, investigations were mainly focused on the recovery of heat released from air conditioning systems [183]. However, investigations into heat recovery were more diversified in cold regions. Killander and Bass [184] developed and tested a prototype of a thermoelectric generator designed to supply small amounts of electricity using heat from existing wood stoves in homes in the cold and isolated regions of northern Sweden. The cost of connection to the grid in this region ranged from USD 5000 to USD 120,000 per house. The device provided sufficient energy for electric lighting and watching television during the long winter nights. In the Netherlands, Gasunie Research, an energy network operator developed in 1999–2000, built and tested 20 autonomous (self-powered) boilers that used the heater flame produced to co-generate enough electricity to operate its electrical components, using six Hz-20 thermoelectric modules [185]. They concluded that these thermoelectric generators supplied 60 W of electricity, which was enough to run their electrical components. Other tests were carried out in the United States of America and England [186], on residential-scale hydronic central heating units that were modified with the addition of a thermoelectric production stage, to demonstrate autonomous operation in a realistic environment. The thermoelectric stage is a set of 18 thermoelectric modules made of bismuth telluride alloy, which generates an electrical power of 109 W, sufficient to supply the blower, the gas control and the water pump of the hydronic central heating. According to Sornek et al. [187], the commercial success of such an installation has to be focused on: (i) introduction of necessary modifications to the heating devices, and (ii) the development of a dedicated structure of the TEG. In the case of operating conditions in which ambient temperature is not as important as the arrangement of the system off-grid, Bass and Farley [188] tested three thermoelectric generators designed to provide electrical energy in a natural gas field. These generators converted the waste heat produced by the equipment used in the gas field into a thermal energy source for the generators. Electricity was used for cathodic protection, telemetry power supply and lighting. Further, the US military used thermoelectric technology to reduce the logistics of field feeding by integrating thermoelectric devices into the Assault Kitchen that was used to heat food rations out on the field. These devices had no need for an external electrical generator to power the Assault Kitchen. In addition, they produced an excess of electricity that could be used for lighting, battery charging, radio power, communication equipment, etc. [189].

5.3.2. Waste Heat Recovery from Transport Systems

Automobiles Road transport in Europe contributes about 20% of the total carbon dioxide emissions, 75% of which come from private cars, and similar rates are observed in America and Asia [190]. European regulations aim to achieve a CO2 emissions target of 95 g/km by 2021, and 68 g/km by 2025, for passenger cars and light commercial vehicles [191]. It is worth taking into account that two thirds of a vehicle’s combustion energy is lost as waste heat, 40% of which is in the form of hot exhaust gas [192,193]. If about 6% of exhaust heat could be converted to electrical energy, it would be possible to reduce fuel consumption by about 10% [194]. To this end, the major American, Asian and European automobile companies, in collaboration with research institutes and universities, are trying to develop various types of TEGs to improve the fuel economy of their vehicle models, to preserve and gain an additional share in the future automobile market, which will undoubtedly be more restrictive. Agudelo et al. [195] tested a diesel passenger car in a climatic chamber in order to determine the potential for energy recovery from exhaust gases. They concluded that the potential fuel savings ranged from 8% to 19%, and the silencer showed the highest energy losses, so the installation of a TEG needed to be located prior to it. Moreover, there are three main possible locations for the TEG [190], namely: (i) The TEG is placed at the end of the exhaust system; (ii) The TEG is located between the catalytic converter and the silencer—the best Energies 2020, 13, 3606 19 of 32 option; and (iii) The TEG is located upstream of the catalytic converter and silencer. If the weight of the installed TEG and the additional pressure drops in the exhaust system are not optimized, the vehicle will consume more fuel than it needs to save, and consequently the system becomes totally inefficient [196]. The different thermoelectric generators manufactured for automobiles (ATEG) can be compared depending on the shape, material or suitable heat transfer system. In the late 1980s, Birkholt [197], in collaboration with Porsche, proposed a thermoelectric generator with a rectangular cross-section, which was able to produce up to 58 W under peak conditions with FeSi2 elements. At the end of the 1990s, the Nissan Research Center from Japan [198] developed a TE generator with a rectangular cross-section of 72 modules. Each one of these modules contained eight pairs of Si-Ge elements to be applied in gasoline-powered vehicles. The electrical power supplied by the generator was 35.6 W. Later, they tested a thermoelectric generator composed of 16 Bi2Te3 modules operating at low temperature; the electrical power generated by the generator was 193 W [199]. In 1992, Hi-Z Technology started the development of a 1-kW thermoelectric generator for diesel truck engines with funding from the U.S. Department of Energy and the California Energy Commission [200]. Amerigon (now Gentherm) developed thermoelectric generators for passenger vehicles between 2004 and 2011, in five phases. The project was sponsored by the US Department of Energy (DOE), and counted on the participation of BMW and Ford in phase 3, and Faurecia in phase 5. [201]. Phases 1 and 2 dealt with the tests of a low-temperature liquid/liquid TEG, which developed 500 W and was built with Bi2Te3 materials implanted in a flat plate TEG design [201,202]. In phase 3, a two stage flat plate-shaped gas/liquid TEG at a high temperature (above 600 ◦C), consisting of TE elements segmented in two stages and based on a half-Heusler alloy (Zr, Hf), was installed near the hot gas inlet and Bi2Te3 elements near the outlet. The measured output power was about 100 W [201,203]. During phases 4 and 5, a new cylindrical design was selected due to the limitations encountered with the flat plate design, and the power output reached over 200 W in phase 4. In the final phase, power output improvement was achieved with a BMW X6 (Figure 17) and a Lincoln MKT (Ford), with more than 600 W produced in vehicle tests and more than 700 W in bench tests [201,204,205]. This success led Gentherm, BMW and Tenneco in 2011 to launch a new program, projected over seven years, using a new thermoelectric cartridge device. The electricity generated by the TEG could save fuel consumption by 2%, which was far from the program’s aims. General Motors developed a prototype using Bi-Te and Skutterudite modules, which was installed on a Chevrolet suburban [206]. The skutterudites used at high temperatures were their final choice. Average power developed by the TEG was expected to be 350 W for city driving cycles, and 600 W on motorways [207]. In 2013, Fiat and Chrysler announced the manufacture of the first light commercial vehicle equipped with a TEG [208] with a fuel economy of 4%. The TEG used cross-flow architecture, with segmented TE elements of TAGS, Bi2Te3-PTe and Skutterudites. Another relevant development was called RENOTER [209], an association between Renault and Volvo involving eight partners, partially financed by the French government. This project was carried out with the aim of installing thermoelectric generators (the TEGs) in their car range. Bou Nader [211] proposed an innovative thermodynamic configuration, and investigated the fuel-saving potential of hybrid electric vehicles using a thermoelectric generator system as an energy converter, instead of the conventional internal combustion engine. Simulation results revealed a 33% higher fuel consumption with the chosen thermoelectric generator configuration, compared to the conventional internal combustion engine. This investigation highlighted the importance of increasing the thermoelectric generator module’s figure of merit in order to achieve a system efficiency comparable to the internal combustion engine. This energy converter has the potential for implementation in future powertrains with zero carbon alternative fuels. Recently, Shen et al. [210] presented the current status, challenges and future prospects of automotive exhaust thermoelectric generators. The authors cited 11 challenges to overcome before the possibility of commercial applications could be realized. These are mainly the low efficiency of the TEG, insufficient heat extraction capacity and non-uniform temperature distribution on the exhaust side, and the space limitation. Energies 2020, 13, x FOR PEER REVIEW 20 of 34

Tenneco in 2011 to launch a new program, projected over seven years, using a new thermoelectric cartridge device. The electricity generated by the TEG could save fuel consumption by 2%, which was far from the program’s aims. General Motors developed a prototype using Bi-Te and Skutterudite modules ,which was installed on a Chevrolet suburban [206]. The skutterudites used at high temperatures were their final choice. Average power developed by the TEG was expected to be 350 W for city driving cycles, and 600 W on motorways [207]. In 2013, Fiat and Chrysler announced the manufacture of the first light commercial vehicle equipped with a TEG [208] with a fuel economy of 4%. The TEG used cross-flow architecture, with segmented TE elements of TAGS, Bi2Te3-PTe and Skutterudites. Another relevant development was called RENOTER [209], an association between

EnergiesRenault2020 and, 13 ,Volvo 3606 involving eight partners, partially financed by the French government.20 This of 32 project was carried out with the aim of installing thermoelectric generators (the TEGs) in their car range.

Figure 17. TEG integration into the exhaust line of the BMW X6 prototype vehicle [210]. Figure 17. TEG integration into the exhaust line of the BMW X6 prototype vehicle [210]. Motorcycles Bou Nader [211] proposed an innovative thermodynamic configuration, and investigated the fuel-savingMotorcycles potential are theof hybrid most commonly electric vehicles used means usingof a thermoelectric transport in some generator countries, system such as an Indonesia. energy converter,Septiadi et instead al. [212 ]of investigated the conventional the usefulness internal co ofmbustion installing engine. a thermoelectric Simulation generator results revealed on the exhaust a 33% higherof a motorcycle. fuel consumption Their results with showed the chosen that thethermoel outputectric voltage generator reached configuration, was 15.7 V and compared 7.7 V, for to TEGs the conventionalof four modules internal and two combustion modules, respectively. engine. This In 2013, investigation ATSUMITEC, highlighted in cooperation the withimportance the Nagoya of increasingInstitute of Technology,the thermoelectric applied thegenerator Heusler module’s module to figure the underpower of merit in generator order to of aachieve motorcycle a system [213], efficiencyby integrating comparable the thermoelectric to the internal device combustion with a fuelengine. cell, This as illustratedenergy converter in Figure has 18 the. Thepotential fuel cellfor producedimplementation energy in from future trace powertrains quantities ofwith spent zero fuel carbon in the alternative exhaust gases. fuels. The Recently, temperature Shen et di al.ﬀerence [210] Energiespresentedbetween 2020 heat , the13, x fromcurrent FOR PEER the status, exhaust REVIEW challengesgases and and heat future generated prospects by the of chemical automotive reaction exhaust in the thermoelectric fuel cell21 of was 34 generators.used to generate The powerauthors by cited means 11 of challenges the thermoelectric to overcome device. before The total the output possibility power of of commercial the fuel cell reactionplusapplications the thermoelectricin the could fuel cellbe powerrealized.was used was Theseto 400 generate W are [214 mainly ].powe Schlichtingr theby meanslow et efficiency al. of [215 the] thermoelectric tested of the the TEG, feasibility device.insufficient of The installing totalheat outputextractiona TEG power on acapacity motorcycle, of the andfuel non-uniform withcell plus the the aim thermoelectrtemperature of replacingic distribution the power alternator was on400 with the W [214].exhaust a TEG Schlichting unit. side, Theyand et theused al. space[215] 570 testedlimitation.modules the to feasibility match the of alternator installing output. a TEG Theyon a concludedmotorcycle, that with the the potential aim of of replacing replacing the the alternator alternator withwith a TEG TEG units unit. was They quite used low. 570 modules to match the alternator output. They concluded that the potential5.3.2.2. Motorcycles of replacing the alternator with TEG units was quite low. Motorcycles are the most commonly used means of transport in some countries, such as Indonesia. Septiadi et al. [212] investigated the usefulness of installing a thermoelectric generator on the exhaust of a motorcycle. Their results showed that the output voltage reached was 15.7 V and 7.7 V, for TEGs of four modules and two modules, respectively. In 2013, ATSUMITEC, in cooperation with the Nagoya Institute of Technology, applied the Heusler module to the underpower generator of a motorcycle [213], by integrating the thermoelectric device with a fuel cell, as illustrated in Figure 18. The fuel cell produced energy from trace quantities of spent fuel in the exhaust gases. The temperature difference between heat from the exhaust gases and heat generated by the chemical

FigureFigure 18. 18. TEGTEG installed installed in in the the exhaust exhaust gases gases of of a a motorcycle motorcycle [213]. [213].

5.3.2.3.Aircraft Aircraft ModernModern aircrafts aircrafts are are increasingly increasingly equipped equipped with with sensors sensors and and transmitters transmitters for for better better control control and and monitoring,monitoring, and and greater greater safety. safety. The The supply supply of of power power to to these these sensors sensors via via power power lines lines would would result result in in additionaladditional heavy wiring, causingcausing additionaladditional fuel fuel consumption. consumption. The The use use of of thermoelectric thermoelectric generators generators to to power such instruments is one of the most promising approaches. The implementation of autonomous wireless sensor networks would lead to a reduction in aircraft weight and complexity, thus reducing fuel consumption. Boeing Research & Technology estimated that a 0.5% reduction in fuel consumption would be translated into a USD 12.075 million reduction in monthly operating costs for U.S. commercial aircrafts, and approximately a 0.03% reduction in carbon emissions for U.S. passenger aircrafts, taking into account the fact that aircrafts’ contribution to global carbon emissions is approximately 2% [216]. A recent study carried out in the framework of the German Aeronautical Research Program (LuFo-5) on the performance of a thermoelectric generator, integrated between the hot part of a propeller and the bypass flow of the cooler, demonstrated that the efficiency of the TEG ranged from 3% to 7%, with a power of 1 kW/m2 to 9 kW/m2 depending on its location in the various hot parts of the propeller [217]. Lyras et al. [218] designed a TEG to be installed between the inside and outside walls of the aircraft, since the fuselage is exposed to extremely low temperatures (~−50 °C) [219], while the inside of the aircraft has a controlled temperature (~+20 °C) for passenger comfort. Some sensors, such as the stress sensor [220], which controls the state of health of the hull, must be installed on different parts of the aircraft. Therefore, it would be very useful to be able to employ a TEG attached directly to the fuselage and combined with a phase-change material (PCM) heat storage unit. This would create a temperature gradient during takeoff and landing, which could generate electricity to power a node of autonomous low-power wireless sensors [221,222]. The system was successfully integrated and functionally tested, qualifying it for use in a flight test facility [220]. Helicopters have also been considered, through a study on the feasibility of recovering helicopter exhaust gases using thermoelectric modules. The results of this study showed that the electrical energy produced under real operating conditions was significant, but currently insufficient, particularly if we take into account the weight/power ratio [223]. 5.2.3.4. Ships There is scarce research reported regarding the thermoelectric recovery of lost heat from ships in the open literature. Maritime shipping alone represents about 2.8% of greenhouse gases in the world [224]. Moreover, the integration of thermoelectric power generation into ships is more

Energies 2020, 13, 3606 21 of 32 power such instruments is one of the most promising approaches. The implementation of autonomous wireless sensor networks would lead to a reduction in aircraft weight and complexity, thus reducing fuel consumption. Boeing Research & Technology estimated that a 0.5% reduction in fuel consumption would be translated into a USD 12.075 million reduction in monthly operating costs for U.S. commercial aircrafts, and approximately a 0.03% reduction in carbon emissions for U.S. passenger aircrafts, taking into account the fact that aircrafts’ contribution to global carbon emissions is approximately 2% [216]. A recent study carried out in the framework of the German Aeronautical Research Program (LuFo-5) on the performance of a thermoelectric generator, integrated between the hot part of a propeller and the bypass flow of the cooler, demonstrated that the efficiency of the TEG ranged from 3% to 7%, with a power of 1 kW/m2 to 9 kW/m2 depending on its location in the various hot parts of the propeller [217]. Lyras et al. [218] designed a TEG to be installed between the inside and outside walls of the aircraft, since the fuselage is exposed to extremely low temperatures (~ 50 C) [219], while the inside of the − ◦ aircraft has a controlled temperature (~+20 ◦C) for passenger comfort. Some sensors, such as the stress sensor [220], which controls the state of health of the hull, must be installed on different parts of the aircraft. Therefore, it would be very useful to be able to employ a TEG attached directly to the fuselage and combined with a phase-change material (PCM) heat storage unit. This would create a temperature gradient during takeoff and landing, which could generate electricity to power a node of autonomous low-power wireless sensors [221,222]. The system was successfully integrated and functionally tested, qualifying it for use in a flight test facility [220]. Helicopters have also been considered, through a study on the feasibility of recovering helicopter exhaust gases using thermoelectric modules. The results of this study showed that the electrical energy produced under real operating conditions was significant, but currently insufficient, particularly if we take into account the weight/power ratio [223].

Ships There is scarce research reported regarding the thermoelectric recovery of lost heat from ships in the open literature. Maritime shipping alone represents about 2.8% of greenhouse gases in the world [224]. Moreover, the integration of thermoelectric power generation into ships is more advantageous than integration into other transportation systems, because cooling water is fully available. The integration of thermoelectricity in this sector is almost inexistent because of the absence of strict international regulations imposing permissible pollution rates for ships, unlike the automotive sector. The European Union is currently planning measures to reduce emissions from international maritime transport [22], so the introduction of new and stricter rules should increase the rate of research. A project called ECOMARINE, co-financed by the European Union (European Social Fund) and Greek national funds, was carried out to implement a thermoelectric energy recovery unit, with the aim of maximizing electricity production by waste heat recovery, and simultaneously improving the quality of electrical energy. In this context, Loupis et al. [225] developed a tubular TEG with a diameter of 500 mm, which ensured a very low pressure drop of the exhaust gas flows during their passage through the RTG. The authors reported a conversion efficiency of 6.4%, a waste heat recovery of 1.2%, and an electricity supply of 20.3 kW.

6. History of International Conferences on Thermoelectric The first International Conference on the Conversion of Thermoelectric Energy (ICOTEC) was held in 1976, and afterwards every two years until 1988 at the University of Texas at Arlington. In 1988, the European Conference on Thermoelectricity and the International Conference on the Conversion of Thermoelectric Energy were merged, and called the International Conference on Thermoelectricity (ICT). Since then, this conference has been organized every two years in Europe and the United States of America. At the ICT1990 conference held in Pasadena, California, Charles Wood was nominated as the first president of the International Thermal Electricity Society (ITS), and placed in charge of the Energies 2020, 13, 3606 22 of 32 coordination of thermoelectricity activities worldwide. In 1993, the ICT conference was organized in Japan, and a three-year rotation is taking place between the United States, Europe and Asia [226]. The European Thermoelectric Society (ETS) is a non-profit scientific association, founded in 1995 as a regional organization of the International Thermoelectric Society (ITS). It organizes the annual European Conference on Thermoelectricity (ECT). The 19th ECT and 40th ICT will be held in Krákow, Poland in 2021. Details about the previous conferences and workshops can be found on the ETS web page in reference [227–230].

7. Conclusions The special nature of using thermoelectric generators, to supply an electric current with a temperature difference of any small value and over a wide temperature range, has made them the core solution to certain energy problems regarding power generation and heat recovery in a static and non-polluting way, even under extreme environmental conditions. The low efficiency of this conversion technology has limited its development, except in certain sectors where the advantages of TEGs are more favorable over other technologies. The use of thermoelectricity in various laboratory and industrial sectors has resulted in there being different perspectives. It has achieved significant success in some applications and total failure in others. The current investigations into thermoelectric generators are focused on the development of new efficient thermoelectric materials to overcome the drawbacks of the interconnected electrical and thermal properties of these materials, and new designs of thermoelectric generators that allow better integration into energy conversion systems, from the point of view of efficiency and the environmental impact. Interest in this technology has been revived with the appearance of nanotechnology, which has made it possible to cross the historical barrier of ZT<1, resulting in an exponential increase in publications in this field. In this review, the state-of-the-art of thermoelectric generators, applications and recent progress are all reported. Fundamental knowledge of the thermoelectric effect, basic laws, and parameters affecting the efficiency of conventional and new thermoelectric materials, are all discussed. The applications of thermoelectricity are grouped into three main domains. The first group deals with the use of heat emitted from a radioisotope to supply electricity to various devices, with only space proving to be the area in which thermoelectricity was successful. In the second group, a natural heat source can be useful for producing electricity, but this group of thermoelectricity is still at an undeveloped phase because of low conversion efficiency, leaving applications still at laboratory level. The third group is progressing at a high speed, mainly because the investigations are funded by governments and/or car manufacturers whose final aim is to reduce vehicle fuel consumption, and consequently mitigate greenhouse gas emissions.

Author Contributions: Investigation, methodology and writing—original draft preparation, M.A.Z.; conceptualization, supervision and funding acquisition, S.B; conceptualization and supervision, J.G.S.; conceptualization, methodology, writing—review and editing and supervision, M.B. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: Mohamed Amine Zoui gratefully acknowledges the University of Adrar for funding his internships at Rovira i Virgili University of Tarragona (Spain). Conﬂicts of Interest: The authors declare no conﬂict of interest.

Nomenclature

ZT ﬁgure of merit 1 S Seebeck coeﬃcient, µV/K− 1 σ electrical conductivity, S/m− 1 1 K thermal conductivity, W/m− /K− T temperature, K Energies 2020, 13, 3606 23 of 32

∆T temperature diﬀerence, K Th temperature on the warm side, K Tc temperature on the cold side, K Ci non-SI unit of radioactivity Greek letters ηopt eﬃciency

References

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